Stretch-reducing mill having improved diameter tolerance and wall thickness tolerance

ABSTRACT

A stretch-reducing mill ( 1 ) for producing seamless pipes (R), which comprises a plurality of roll stands ( 10 ), which are arranged one after the other in a conveying direction (F) of the pipes (R) and each have three rolls ( 11 ) arranged at an angular distance of 120°, wherein the roll stands ( 10 ) are divided into at least two groups (A, B) each having at least two roll stands ( 10 ); the rolls ( 11 ) of adjacent roll stands ( 10 ) within a group (A, B) are inclined relative to each other by an intra-group angle αI; and the rolls ( 11 ) of the roll stands ( 10 ) of adjacent groups (A, B) are inclined relative to each other by a group angle αG that is less than the intra-group angle αI.

TECHNICAL FIELD

The invention relates to a stretch-reducing mill for the production of seamless tubes, which has a plurality of roll stands arranged one behind the other in a conveying direction of the tubes, each with three rolls arranged at an angular distance of 120°.

BACKGROUND OF THE INVENTION

In the manufacture of seamless tubes, stretch-reducing and/or sizing mills are used which have several rolling stands arranged one behind the other in the conveying direction of the tube. The roll stands usually have three rolls which are arranged symmetrically about the pipe at an angular distance of 120°.

It is known to arrange adjacent roll stands offset by 60° relative to one another, so that sections of the tube to be rolled are rolled alternately in the caliber base and from the caliber jump of the rolls. Thus, WO 2017/068533 A1 describes a rolling mill with a first section, which has several roll stands and is set up for rolling via a mandrel introduced into the tube, and a second section, which has several roll stands and is set up for rolling without a mandrel. The roll stands each have three rolls. The axes of rotation of the rolls of adjacent roll stands are each tilted by 180°, as shown in FIGS. 1A and 1B of WO 2017/068533 A1. This corresponds to an offset by the above-mentioned 60°.

Especially in the case of thick-walled pipes in connection with large diameter reductions, the inside of the pipe is unevenly formed in the stretch-reducing mill due to the uneven speed distribution between the caliber base and the caliber jump of the rolls. A polygon-like internal cross-section is formed perpendicular to the pipe axis. This phenomenon is also known as “inner polygon formation”. When the rolling mill functions as an extraction mill, additional complications result from temperature differences from the preceding stretching unit, which also cause different forming conditions between the caliber base and the caliber jump. This is superimposed on the inner polygon formation of the following aggregate and can thus intensify the effect. The internal polygon formation is particularly pronounced when both the stretching unit and the extraction mill have a three-roller configuration. A four-roller design as an alternative is mostly out of the question due to the limited installation space for the roller bearings and the associated low capacity to absorb the forming forces.

SUMMARY OF THE INVENTION

An object of the invention is to improve the rolling quality of a stretch-reducing mill for the production of seamless tubes, in particular to equalize the rolled wall thicknesses.

The object is achieved with a stretch-reducing mill with the features of claim 1. Advantageous further developments follow from the dependent claims, the following description of the invention and the description of preferred exemplary embodiments.

The stretch-reducing mill according to the invention is used to produce seamless tubes, preferably from a metal material. The term “stretch-reducing mill” is to be understood here as a generic term for rolling mills which bring about both a reduction in the outside diameter of the tube and a reduction in the inside diameter, resulting in elongation of the tube. For this reason, the stretch-reducing mill is preferably a mandrel-less mill. Depending on the set roll speeds, the wall thickness of the pipe can also be increased or decreased to a certain extent. The stretch-reducing mill has a plurality of roll stands arranged one behind the other in the conveying direction of the pipes, each with three rolls arranged at an angular distance of 120°. The rollers are thus arranged symmetrically about the pipe in order to exert a rolling force on the outer circumference of the pipe from three sides. The roll stands are subdivided into at least two groups, each with at least two roll stands, the rolls of adjacent roll stands within a group being inclined relative to one another at a group internal angle. Furthermore, the rolls of roll stands of adjacent groups are inclined relative to one another at a group angle that is smaller than the group internal angle.

The term inclined includes a relative rotation of adjacent roll stands (analogous to groups)—more precisely, their rollers—by the said group-internal angle (analogous to group angles). However, since the rollers are arranged symmetrically at an angular distance of 120° about the pipe, a state which corresponds to a rotation by a certain angle can also be achieved by rotation by one or more other angles. For example, an inclination of 60° can also be achieved by tilting by 180°. For this reason, the term “inclined” at an angle is used herein to denote a rotation by that angle as well as all equivalent angles. The rotation can take place both clockwise and counterclockwise in relation to the rolling direction.

According to the invention, the rolling stands are inclined in a group-related manner by a group angle which is smaller than the group-internal angle. Due to such an inclination, the inner polygon formation of a group is superimposed with an inner polygon formation of the following group inclined by the group angle. This improves approximation to a circular inner cross-section of the pipe. Another technical effect is that the group-wise angular offset of the rollers improves the temperature equalization in the pipe, provided there is a temperature gradient along the radial direction of the pipe. Both effects contribute to the equalization of the rolled wall thicknesses and thus to the improvement of the rolling quality when rolling seamless tubes.

The group-internal angle is preferably 60°, which results in sections of the tube being alternately rolled in the groove base or from the groove jump. This improves the wall thickness characteristics within the group.

The group angle is preferably 60°/n, where n is an integer greater than 1, i.e., n=2, 3, 4 . . . . The group angle is preferably 30°. Due to such an inclination, the inner polygon formation of a first group is superimposed with an inner polygon formation inclined by 30° in a subsequent second group. A dodecahedron-shaped inner polygon is generated whose deviations between a maximum wall thickness and a minimum wall thickness are significantly reduced compared to a hexagonal polygon.

It should be pointed out that the number of rolls per caliber can in principle differ from “three”; in particular, four rolls per caliber are possible, even if this tends to be the exception in practice. In the case of four rolls per caliber, the group-internal angle is preferably 45° and the group angle is preferably 45°/n, where n is an integer greater than 1, i.e., n=2, 3, 4 . . . .

The rolls of the group-related roll stands preferably have a caliber shape that deviates from the circular shape. This allows preventing material from entering the gap between the rolls, which may damage the surface of the rolling stock.

Preferably, between two groups (when viewed in conveying direction of the pipe) there is at least one neutral roll stand with three rolls each arranged at an angular distance of 120°, the shape of which counteracts a torsional moment acting on the pipe. The neutral roll stand thus serves preventing the pipe from twisting between two adjacent groups. The cause of potential twisting of the tube is that, especially in the case of non-circular calibres, a torsional moment can act on the tube about its own axis if a group-related inclination is used. In order to counteract such a tendency to torsion, at least one neutral roll stand is preferably connected between adjacent groups. A neutral roll stand can be characterized, for example, in that its caliber shape deviates less from the circular shape than that of the other stands and/or the decrease in diameter is reduced relative to the other stands. The rolls of the one or more neutral roll stands preferably have a circular or approximately circular caliber shape.

The one or more neutral roll stands preferably do not form (an) independent group(s), rather they are preferably completely or at least partially a structural component of the groups defined above, for example the group located upstream in the conveying direction. If not all of the neutral roll stands belonging to the transition between two groups are part of the group located upstream, the remaining neutral roll stands are structurally preferably part of the group located downstream. In this way, a special structural solution can be avoided.

The size of the groups, i.e., the number of roll stands in the respective group, can be made dependent on the dimensions to be rolled. Thus preferably about 35% to 70% of the total diameter reduction takes place in the first group and the remaining diameter reduction in the second group. The reason for this distribution is that the inner polygon formation gradually builds up and thus there is a risk of overcompensation. In other words, in the case of an unfavorable distribution, the optimal compensation is not behind the last roll stand, but on an inner roll stand.

The stretch-reducing mill is preferably an extraction mill. An extraction mill means a mill that is located downstream of a stretching mill with a mandrel, in order to pull the pipe off the mandrel after having been rolled by the mandrel. Extracting mills are increasingly designed so that, in addition to the simple separation of the pipe from the mandrel, they also cause a comparatively strong deformation of the pipe. In this process stage, the pipe has a comparatively strong temperature gradient from the outside (cold) to the inside (warm). If the extraction mill is designed as a stretch-reducing mill, the inhomogeneous temperature distribution in the tube can have a particularly detrimental effect on the rolling result. For this reason, a group-related inclination is particularly suitable for an extraction mill designed as a stretch-reducing mill.

Further advantages and features of the present invention will become apparent from the following description of preferred exemplary embodiments. The features described there can be implemented alone or in combination with one or more of the features described above, provided that the features are not incompatible with one another. The following description of preferred embodiments is made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a stretch-reducing mill with grouped rolling stands.

FIG. 2 shows a rolling stand in three-roll design.

FIG. 3 shows schematically a group-wise inclination of roll stands.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

Preferred exemplary embodiments are described below with reference to the Figures. Identical, similar or identically functioning elements are provided with identical reference symbols in the Figures, and a repetitive description of these elements is partially dispensed with in order to avoid redundancies.

FIG. 1 is a schematic view of a stretch-reducing mill 1. The stretch-reducing mill 1 has a plurality, here fifteen, roll stands 10. The roll stands 10 can preferably be controlled individually. In particular, the speeds of the rolls 11 (see FIG. 2) of the roll stands 10 can be set individually.

The rolling stands 10 are controlled via a control device 2, preferably computer-based. The control device 2 takes over the control of further components of the stretch-reducing mill 1 if necessary. It should be pointed out that the term “control device” includes both centralized and decentralized structures for controlling the stretch-reducing mill 1. The control device 2 therefore does not have to be located at the “location” of the stretch-reducing mill 1 or be part of it. In addition, control tasks, data processing steps, etc. can be distributed to different computing devices, which then fall under the term “control device” in their entirety. Furthermore, the communication of the control device 2 with the components to be controlled can take place both physically via cable as well as wirelessly.

To roll a pipe R, the pipe runs in a conveying direction F through the stretch-reducing mill 1. Before entering the stretch-reducing mill 1, the pipe R has an inlet-side wall thickness d1. When exiting the last roll stand 10, the pipe R has a wall thickness d2 and a reduced diameter. The wall thickness d2 is not necessarily reduced compared to the wall thickness d1; rather, depending on the roll speed, it can be smaller, the same, but also larger than the initial wall thickness d1.

The inlet-side and/or outlet-side wall thickness d1 or d2 can be measured by means of one or more wall thickness measuring devices (not shown). In addition, further process parameters can be measured or otherwise determined, for example the inlet-side and/or outlet-side speed of the pipe R, the inlet-side and/or outlet-side weight of the pipe R, etc. The determined process parameters can be transmitted to the control device 2 to control the rolling process.

FIG. 2 shows a roll stand 10, 10′ with three rolls 11 arranged symmetrically about the pipe R at an angular distance of 120°. According to this exemplary embodiment, the rolls 11 each have an arcuate cross section of the roll surface, i.e., a round caliber shape. The caliber base 13 refers to the center of the roll surface, viewed in the cross section of FIG. 2 and in the axial direction of the corresponding roll 11. The two outer ends of the roll surface—also seen in the cross section of FIG. 2 and in the axial direction of the corresponding roller 11—are referred to as the caliber jump 14. The caliber base 13 and the caliber jump 14 are characteristic positions of the roll surface.

The shape of the caliber preferably deviates from a perfect circular arc. The reason for the deviation from the circular shape is that in this way material can be prevented from entering the gap between adjacent rolls 11—more precisely, between the caliber jumps 14 of adjacent rolls 11. Local caliber size reduction and local caliber size increase allows compensation of deviations in the pipe diameter.

Referring again to FIG. 1, the roll stands 10 are divided into two groups A and B, each of which has seven roll stands 10. The groups A, B are disjoint, i.e., they are arranged sequentially one behind the other and do not overlap or interpenetrate. Within a group A, B, the rolls 11 of the roll stands 10 according to the present exemplary embodiment are inclined relative to one another by an internal group angle ai of 60° or approximately 60°, so that the tube sections are alternately rolled in the caliber base 13 or in the caliber jump 14. The number of roll stands 10 per group A, B is selected so as to minimize the formation of the inner polygon. The number of roll stands 10 per group A, B is at least two; preferably in the range from 2 to 8. The number of roll stands 10 can vary from group A to group B.

The groups A, B—more precisely, the rolls 11 of the roll stands 10 of two, preferably adjacent groups A, B—are inclined relative to each other by an angle which is referred to herein as the group angle OG. Preferably, OG is equal to or about 30°. As a result of the inclination, the inner polygon formation of a group A is superimposed with an inner polygon formation inclined by OG of the following group B. To a certain extent, an inner dodecahedron is generated, which deviates between a maximum wall thickness and a minimum wall thickness significantly less compared to an inner hexagon. The internal geometry of the pipe R is thus approximated to an ideal round.

The group-related rotation or inclination described above is shown schematically in FIG. 3. The roll stands 10 of groups A, B are shown schematically in FIG. 3. Within a group A, B, the roll stands 10 or their rolls 11 (without reference numerals in FIG. 3) are alternately rotated or tilted by 180°, whereby the above-described inclination of 60° is achieved. A rotation of 90° was made between groups A and B, resulting in an inclination of 30°. Of course, the two types of inclination can also be achieved directly by rotating about the group angle OG and the group-internal angle ai. The variant shown in FIG. 3, however, has structural advantages, in particular when coupling the rolls 11 to the drives (not shown). More precisely, when using roll stands 10 each with three rolls 11, the offset of 60° results, for example, by tilting the roll stands 10 by 180°. As a result, the drives can be arranged on the usual feed-in side and opposite the feed-in side with internal power distribution. This simplifies the installation and maintenance of the drives, since the coupling can take place directly when the scaffolding is pushed in.

The size of the groups A, B, i.e., the number of roll stands 10 in the respective group A, B, can be made dependent on the dimension to be rolled. Thus, preferably about 35% to 70% of the total diameter reduction takes place in the first group A and the remaining diameter reduction in the second group B. The reason for this distribution is that the inner polygon formation gradually increases and thus there is a risk of overcompensation. In other words, in the case of an unfavorable distribution, the optimal compensation is not downstream of the last roll stand 10, but on an inner roll stand 10.

FIGS. 1 and 4 also show a roll stand 10′, which is referred to herein as a “transition pass” or “neutral roll stand”. The neutral roll stand 10′ serves to avoid twisting of the pipe R between groups A and B. The cause of any twisting of the pipe R is that in the case of non-circular deformation, a torsional moment can act on the pipe R about its own axis if a group-related inclination of OG is applied. In order to counteract such a tendency to torsion, at least one neutral roll stand 10′ is preferably connected between groups A, B. The caliber shape of the rolls 11 of the neutral roll stand 10′ is thus determined so as to counteract the tendency to torsion. The rolls 11 of the neutral roll stand 10′ preferably have a circular or approximately circular caliber shape, as shown in FIG. 2.

The group-related inclination about the group angle OG, as described above, results in an improved approximation of the the internal geometry of the tube R to an ideal circular cross-section.

Another technical effect is that the caliber base 13, which is offset in groups, improves temperature compensation in the event of a temperature gradient in the pipe R. This effect particularly comes to bear in the case of inhomogeneous temperature distribution along the radial direction of the tube R, as is the case in particular in extraction mills. Extracting mills, which are normally located immediately downstream of a stretching mill with a mandrel, serve to separate the tube R from the mandrel. In this process stage, the tube R has a comparatively strong temperature gradient from the outside (cold) to the inside (warm). If the extraction mill is designed as a stretch reducing mill 1, i.e., in addition to the separation of mandrel and tube R, it is also designed for strong deformation of the tube R, the inhomogeneous temperature distribution in the tube R can have a detrimental effect on the rolling result. For this reason, a group-related inclination according to the disclosed exemplary embodiments, is particularly suitable for an extraction mill, in particular an extraction rolling mill designed as a stretch-reducing mill 1.

As far as applicable, all of the individual features set out in the exemplary embodiments can be combined with one another and/or exchanged without departing from the scope of the invention.

LIST OF REFERENCE NUMERALS

-   1 stretch-reducing mill -   2 control device -   10 roll stand -   10′ neutral mill stand -   11 roller -   13 caliber base -   14 caliber jump -   A group of roll stands -   B group of rolling stands -   R pipe -   F direction of travel -   d1 wall thickness on the inlet side of the pipe -   d2 Wall thickness on the outlet side of the pipe Qi Group-internal     angle -   OG group angle 

1. Stretch-reducing mill (1) for the production of seamless tubes (R), comprising: plural roll stands (10) arranged one behind the other in a conveying direction (F) of the tubes (R), each of said roll stands having three rolls (11) arranged at an angular distance of 120°, wherein the roll stands (10) are divided into at least two groups (A, B) each including at least two roll stands (10), the rolls (11) of adjacent ones of the roll stands (10) within a group (A, B) are inclined relative to one another by an angle αI within the group, and wherein the rolls (11) of the roll stands (10) of adjacent groups (A, B) are inclined relative to one another by a group angle αG which is smaller than the group-internal angle αI.
 2. Stretch-reducing mill (1) according to claim 1, characterized in that the group-internal angle αI is approximately 60°.
 3. Stretch reducing mill (1) according to claim 1, characterized in that the group angle αG is 60°/n, where n is an integer greater than
 1. 4. Stretch reducing mill (1) according to claim 3, characterized in that the group angle αG is approximately 30°.
 5. Stretch reducing mill (1) according to claim 1, characterized in that the rolls (11) of the roll stands (10) of the groups (A, B) have a caliber shape deviating from the circular shape.
 6. Stretch-reducing mill (1) according to claim 1, characterized in that between two groups (A, B) at least one neutral roll stand (10′) is provided, each with three rollers (11) arranged at an angular distance of 120°, their caliber shape counteracts a torsional moment acting on the tube (R).
 7. Stretch reducing mill (1) according to claim 6, characterized in that the caliber shape of the rollers (11) of the neutral roll stand (10′) is in the shape of a segment of a circle.
 8. Stretch reducing mill (1) according to claim 1, characterized in that 35% to 70% of the total diameter reduction of the tube (R) takes place in the first group (A) and the remaining diameter reduction in the second group (B).
 9. Stretch reducing mill (1) according to claim 1, characterized in that this is an extraction mill. 